The University Group Diabetes Program (UGDP) was undertaken for the purpose of evaluating the effect of hypoglycemic treatment on the rate of development of vascular complications in mild adult-onset diabetes. It is a multi-clinic study involving twelve clinical centers and a coordinating center. The first grants supporting this study were awarded in 1960. Patient recruitment for this study started in February 1961 and was completed in early 1966 after a total of 1,027 patients had been enrolled. Patients were randomly assigned to one of the following treatment groups: insulin given in fixed dose, insulin given in variable dose to maintain blood glucose levels within the normal range, tolbutamide, phenformin or placebo. All patients were placed on diets designed to achieve and to maintain normal body weight. Information obtained from this study will be used to evaluate the efficacy of the treatment under study in preventing or delaying the onset and development of vascular complications in mild diabetics and to provide detailed information on the natural history of diabetes.